The decay of perishable agricultural produce is caused by microbial infection. Such produce is typically kept for long enough periods of time during which conditions allowing for the propagation of various microorganisms exist and accordingly, very often, a high percent of the produce becomes infected. In addition to the obvious substantial financial loss due to such decay, some of these microorganisms produce toxic and carcinogenic metabolites, which are harmful to humans.
Control of pathogen infection of perishable agricultural produce is achieved today mainly by exogenous application of synthetic fungicides and/or bactericides. However, these synthetic chemicals have toxic residues in the produce. Furthermore, development of resistant strains of microorganisms has also been observed. As a result, several such fungicides and bactericides are being phased out by regulating agencies. The residual toxicity and the potential phasing out gave rise to the development of alternatives to the synthetic chemicals presently used for prevention of decay.
Several of the alternatives are described below. For example, irradiating the agricultural produce by ultraviolet light (Ben-Yehoshua, S., Rodov, V, Kim, J. J. and Carmeli, S., 1992. Preformed and induced antifungal materials of citrus fruits in relation to the enhancement of decay resistance by heat and ultraviolet treatments. J. Agric. Food Chem., 40:1217-1221; Rodov, V., Ben-Yehoshua, S., Kim, J. J., Shapiro, B. and Ittah, Y, 1992. Ultraviolet illumination induces scoparone production in kumquat and orange fruit and improves decay resistance. J. Amer. Soc. Hortic. Sci, 117:188-192), or exposing the produce to antagonistic yeasts (Wilson, C. L. and Chalutz, E., 1989. Postharvest biocontrol of Penicillium rots of citrus with antagonistic yeasts and bacteria. Scientia Horticulturae, 40: 105-112). However U.V. irradiation may be phytotoxic and the biocontrol with the antagonistic yeast is not yet well accepted commercially, possibly because of inadequate control of the pathogens. Furthermore, these methods have various drawbacks and some of the relevant health authorities have not yet approved some of them.
Citrus fruit, as well as various other plants, possess some endogenous resistance against pathogens owing to the production of anti-microbial substances in the plant tissues {Ben-Yehoshua, S., Rodov, V, Kim, J. J. and Carmeli, S., (1992) Preformed and induced antifungal materials of citrus fruits in relation to the enhancement of decay resistance by heat and ultraviolet treatments. J. Agric. Food Chem., 40:1217-1221; Ben-Yehoshua, S., Rodov, V, Fang, D. Q., and Kim, J. J., (1995) Preformed antifungal compounds of citrus fruit: effect of postharvest treatments with heat and growth regulators. J. Agric. Food Chem. 43: 1062-1066; Rodov, V., Ben-Yehoshua, S., Fang, D. Q., and Kim, J. J., (1995) Preformed antifungal compounds of lemon fruit: citral and its relation to disease resistance. J. Agric. Food Chem. 43: 1062-1066). It has been previously shown that these substances include essential oil components, which exhibit a broad range of anti-microbial activity. U.S. Pat. Nos. 5,334,619 and 5,958,490 describe the use of several natural occurring oils as active agents for preventing decay in post-harvest agricultural products. However only few of the essential oil components have microbiocidal activity.
Citral [3,7-dimethyl-2,6-octadienal] is an essential oil component which is naturally produced in several kinds of citrus fruits as well as in some other plants such as lemon grass and eucalyptus. Citral is an unsaturated aldehyde from the terpene series and is composed of an isomeric mixture of geranial and neral. Because of its intense lemon aroma and flavor, citral has been extensively used in food and cosmetic industries. Citral is recognized as a safe food additive and is approved for use in food by the U.S. Food and Drug Administration. Citral has also been shown to exhibit a very effective and broad range of antimicrobial and antifungal activity. In fact, Ben Yehoshua et al (1992) and Rodov et al (1995) have shown that citral is the most active constitutive antifungal compound in lemon fruit.
Limonene, 1-methyl-4-(1-methylethenyl)cyclohexene (known also as p-mentha-1,8-diene) is another example of an abundant essential oil component, is which may be extracted from glands of flavedo of citrus fruit. U.S. Pat. No. 4,379,168 and U.S. Pat. No. 5,951,992 describe the use of limonene as an insecticide and pesticide, respectively. However in its pure form it has very low antifungal activity. Chalchat et al. (Chalchat, J. C., Chiron, F., Garry, R. Ph. and Lacoste (2000, J. Essent. Oil Res. 12, 125-134) disclose antimicrobial activity of limonene hydropeoxide against Human pathogens.
Aureli et al. (Aureli, P., Costantini, A. and Zolea, S., 1992. Antimicrobial activity of some plant essential oil against Listeria monocytogenes. J Food Protection, 55:344-348) showed that some essential oil components have strong activity against pathogenic bacteria such as Listeria and suggested their use in preventing the infection of food by Listeria. 
Several attempts have been made to use citral to control decay of various agricultural produce. It was shown that Citral could reduce grain deterioration of Aspergillus inoculated high moisture barley (Nandi, B., Thomke S. and Fries, N., 1977. Preservation of high moisture barley grains with citral and allyl caproate and preliminary acceptability tests with piglets. Acta Agric. Scand, 27:105-109), rough rice (Mallick, A. K. and Nandi, B., 1982. Deterioration of stored rough rice. IV. Preservation and palatability of citral and propionic acid treated grains. Acta Agric. Scand., 32:177-187) and wheat (Ghosh, J. and Nandi, B., 1985. Preservation of high moisture wheat by some antifungal volatile compounds and palatability tests with rats. Acta Agric. Scand., 35:245-254). Arora and Pandey (Arora, R. and Pandey, G. N., 1977. The application of essential oils and their isolates for blue mold decay control in Citrus reticulata Blanco. J. Food Sci. and Tech 14:14-16) reported that citral, geraniol and other essential oil compounds reduce the blue-mold decay of Citrus reticulata fruit. The inventor of the present invention (Ben-Yehoshua, S., Rodov, V., Kim, J. J. and Carmeli, S., 1992. Preformed and induced antifungal materials of citrus fruits in relation to the enhancement of decay resistance by heat and ultraviolet treatments. J. Agric. Food Chem., 40:1217-1221) showed that application of exogenous Citral to Penicillium-inoculated lemons significantly inhibited their decay.
In most cases of the prior use of essential oil components to prevent decay of agricultural produce, the essential oil component was applied to the produce in an aqueous emulsion. Although partial prevention of the produce's decay was achieved by the use of such substances, essential oil components, including citral and geraniol, are still not used commercially for decay control of perishable agricultural produce. One main reason for not using those substances is that their application to perishable produce, in a concentration effective against the microorganisms, inflicts damage to the produce, which may cause decay later on. For example, essential oils inflict peel damage in fruit and color changes in meat. This damage may be severe and results in a significant decay of the treated produce after a relatively short period of time. Another reason for the lack of their commercial use is their instability as many of these essential oils are unstable and tend to decompose prior to carrying out their bactericidal activity.